Oil Rigs May Turn Migratory Birds Into Shark Food

In the stomachs of tiger sharks in the Gulf of Mexico, researchers are finding something unusual: land-faring migratory birds.

Sharks are known to eat seafaring birds, but land birds such as woodpeckers, meadowlarks, swallows and tanagers are unexpected.

“We’re the first to look this exhaustively at the diet of tiger sharks, as far as I know, and this certainly seems surprising,” said fisheries ecologist Marcus Drymon, leader of an ongoing tiger shark diet study at the Dauphin Island Sea Lab in Alabama.

The American Bird Conservancy thinks the research suggests oil platforms are to blame for dropping migratory birds into Gulf waters. Night-flying birds are known to get trapped in bright light sources, including offshore oil platforms and the 9/11 memorial lights. More than 6,000 illuminated platforms that pepper the Gulf could become giant nighttime bird lures, causing birds to circle in confusion until they’re exhausted, drop into the water and become shark food.

“Unfortunately, we don’t have good baseline data to know that,” Drymon said. “We don’t know if migratory birds are normal things for them to eat or not. There’s no data on tiger shark diet from 100 or 50 or even 30 years ago.”

Migratory land birds in the U.S. typically overwinter in South America and return in the spring or summer. Each leg of the migration is a non-stop flight covering hundreds or even thousands of miles. Threats range from storms and airborne predators to exhaustion and malnutrition.

The birds stay on course in part by using moonlight and starlight to calibrate internal compasses. Human light pollution can interfere with that ancient system, and poses an evolutionarily unique challenge that researchers have just started to study.

While it will take time to determine whether migratory birds were common tiger shark fare prior to the advent of artificially illuminated offshore structures, a 2007 study of birds circling North Sea oil platforms suggests a cheap and effective solution to the potential problem.